1. Field of the Invention
The present invention relates to a method of manufacturing a stator core of an electric rotating machine by thinning outer end portions of steel sheets of the core and performing ironing for the outer end portions.
2. Description of Related Art
A small-sized electric rotating machine efficiently generating a rotational force or electric power has been recently desired. In an alternator representing the machine, a stator core formed of a lamination of steel sheets is disposed, and a conductor is wound on the stator core so as to be received in slots of the core and to be protruded from both ends of the slots as coil ends. To manufacture a small-sized alternator efficiently generating electric power, it is required to densely dispose the conductor in the slots and to densely form the coil ends. Therefore, the number of slots has been inevitably increased while the intervals of the slots are narrowed.
Further, to efficiently generate electric power in the alternator, the strength of the magnetic field induced in the stator core has been heightened. In this case, iron loss in the core caused during the operation of the alternator is undesirably increased. More specifically, a magnetic field is induced in a rotor and a stator in response to an alternating current flowing in the rotor, and the induced direction of this magnetic field is changed in a cycle equal to that of the alternating current. In the core of the stator, eddy currents are produced in response to the changing magnetic field, so that iron loss is caused by the eddy currents in the stator core. The magnitude of the iron loss is proportional to the strength of the magnetic field, the second power of the cycle of the magnetic field, and the second power of the thickness of the steel sheet. When the cycle of the magnetic field is reduced by lowering the rotation speed of the alternator, the level of the electric power is undesirably lowered. Therefore, to reduce the iron loss in the stator core while maintaining the electric power, it is required to reduce the thickness of the steel sheet. However, when the steel sheet is thinned, the mechanical strength of the steel sheet is lowered. In this case, the steel sheet is easily deformed or bent along the axial direction of the stator core.
A helically-laminated core has been generally used as a stator core of the electric rotating machine. To form this laminated core, a long steel plate having magnetic pole teeth on one side of the sheet is press-formed, and the steel plate is helically wound to obtain a plurality of ring-shaped steel sheets disposed in layers as a cylindrical-shaped laminated core. Each ring-shaped steel sheet has the magnetic teeth and slots alternately arranged along the circumferential direction of the sheet on the inner side of the sheet. Further, before the long steel plate is wound, the other side portion of the plate is thinned by a roller or the like so as to lengthen the peripheral portion of each steel sheet. Therefore, the long steel plate can be easily wound.
After the laminated core composed of the ring-shaped steel sheets is obtained, positions of the sheets are minutely adjusted in each of the radial and circumferential directions of the lamination so as to align the sheets within a permissible range in the radial and circumferential directions. Then, the peripheral portions of the sheets are partially welded to one another to fix the positional relationship of the sheets aligned in the radial and circumferential directions. Then, in a finishing process, ironing (or shaving) is performed for the laminated core to adjust the circularity of the core at the outer diameter, the concentricity between inner and outer portions of the core, and the perpendicularity of each sheet to the axial direction of the core. Therefore, the stator core is formed from the laminated core in the ironing. This ironing is, for example, disclosed in Published Japanese Patent First Publication No. 2006-246586.
In this Publication, a stator core forming apparatus is disclosed. This apparatus forms a stator core by performing the ironing for a laminated core. To form the stator core, this apparatus has a work receiving member holding the laminated core, a first pressing member supporting the work receiving member, an inner diameter forming member fixedly disposed to align and reshape inner portions of steel sheets of the stator core, a pushing member pushing the laminated core to the work receiving member to move the laminated core with the work receiving member, a slot correcting member correcting slots of the stator core, a second pressing member moving the pushing member and the slot correcting member, an outer diameter forming member holding the pushing member and the second pressing member and performing ironing for outer portions of the steel sheets of the laminated core, and a third pressing member moving the outer diameter forming member. When the pressing force of the second pressing member is set to be larger than the pressing force of the first pressing member, the laminated core is pushed by the pushing member and is moved down with the work receiving member, the inner diameter forming member aligns and reshapes the inner portions of the laminated core to form the inner portion of the stator core by adjusting the inner diameter of the laminated core. When the pressing force of the third pressing member is set to be larger than the pressing force of the second pressing member, the outer diameter forming member is moved down to perform the ironing for the outer portions of the laminated core. That is, the outer diameter of the laminated core is adjusted, so that the outer portion of the stator core is formed.
In the ironing for the outer portions of the steel sheets composing the laminated core, when an outer die denoting the outer diameter forming member is moved up and down, the outer portions of the laminated core are pushed into an inner hollow of the outer die while receiving the plastic deformation.
Therefore, the inner and outer diameters and the slots of the laminated core can be simultaneously adjusted in one process to form one stator core. Further, a plurality of stator cores can be successively formed. As a result, the cost for the facility forming the stator cores can be reduced, and the processing time required to form the stator cores can be shortened.
However, in the ironing for the outer portions of the steel sheets of the laminated core, because the outer portions are thinned, the outer portions are spaced apart from one another through openings along the axial direction. In this case, before the plastic deformation directed in the radial direction is caused during the ironing in the outer portions in response to the pressing load added to the outer portions, the pressing load initially induces the outer portions to be deformed or bent toward the axial direction of the laminated core. Therefore, the outer portion of a specific steel sheet having a larger outer diameter is easily moved with the outer die so as to reduce the ironing load given to the outer portion, and the outer portion of the specific steel sheet is largely bent so as to discontinuously form a large opening between this sheet and one adjacent sheet.
This large opening (hereinafter, called inter-sheet opening) is not good in view of the external appearance of the stator core. Regarding the quality of the stator core, the stator core is easily rusted at the bent portion forming the inter-sheet opening. Regarding the performance of the stator core, the inter-sheet opening increases the magnetic resistance of the stator core so as to lower the strength of the magnetic field, and electric power outputted from an alternator using this stator core is undesirably lowered. Regarding the mechanical strength of the stator core, when each of though bolts fastens the steel sheets of the stator core to one another along the axial direction of the core through frames attached to respective axial ends of the core, the inter-sheet opening prevents the sheets from uniformly receiving the fastening force. Therefore, it is difficult to reliably hold the steel sheets in the alternator.
The mechanism of generating the inter-sheet opening at the peripheral end of the laminated core will be described with reference to FIG. 1A to FIG. 1C. FIG. 1A is a sectional view of the core forming apparatus disposing a laminated core.
As shown in FIG. 1A, a laminated core 102 helically wound is mounted on a work receiving member 103, a pushing member 107 is moved down to press one axial end of the laminated core 102 and to place the core 102 between the members 103 and 107. Then, a slot correcting member 105 is moved up to align the slots of the core 102 along each of the axial and circumferential directions. Then, an inner diameter forming member 104 is moved up to align and reshape the inner portions of the core 102. Therefore, the sheets of the core 102 are positioned in the apparatus 100 in each of the radial and circumferential directions of the core 102. Then, an outer diameter forming member 106 is moved down.
FIG. 1B is an explanatory view of the ironing not yet performed for outer portions of the core 102, while FIG. 1C is an explanatory view of the ironing now performed for outer portions of the core 102.
As shown in FIG. 1B, the outer portions of the sheets are thinned so as to form a tapered opening between the adjacent outer portions in each pair. Therefore, although the sheets of the laminated core 102 are in contact with one another, the thinned outer portions are spaced apart from one another along the axial direction. Further, because the sheets are positioned so as to adjust the inner circumferential surfaces of the sheets along the axial direction of the core 102, the outer ends of the sheets are placed at various positions in the axial direction, and the outer diameter of each sheet is set at various values along the circumferential direction. Therefore, when the ironing load is given to the core 102 along the axial direction, the ironing load received in the outer portions of the sheets has various magnitudes along each of the axial and circumferential directions.
Therefore, as shown in FIG. 1C, during the ironing for the outer portions of the sheets, because each outer portion facing openings formed on both axial sides thereof is movable along the axial direction, some outer portions are easily deformed and bent along the axial direction in response to the ironing load. In this case, the ironing load used for the plastic deformation directed toward the radial direction is reduced. Further, the bending level of the sheet depends on the length of the outer diameter of the sheet, so that the outer portions of some sheets are bent by various amounts. Therefore, when two sheets adjacent to each other are largely different from each other in outer diameter, the opening between the sheets is enlarged and changed to an inter-sheet opening.
Further, the whole opening along the circumferential direction between two outer portions of the sheets is not changed to this inter-sheet opening, but the inter-sheet opening is discontinuously formed along the circumferential direction. Particularly, the inter-sheet opening is easily formed near the welded point of the outer portions. The reason is as follows. When steel sheets are welded to each other, the hardness in the welded portion becomes higher than that in the non-welded portion, so that the welded portion is hardly bent. In contrast, a non-welded portion placed near the welded portion is easily bent. Therefore, the inter-sheet opening is easily formed near the welded point of the outer portions.
Moreover, as described above, the steel sheets of the stator core have been thinned to reduce the iron loss caused in the core, so that the rigidity or mechanical strength of the steel sheets along the axial direction is inevitably lowered. Therefore, each sheet is further easily deformed and bent in response to the ironing load, and the inter-sheet opening is formed at a higher probability. To manufacture a stator core composed of steel sheets having thinned outer portions, it is important to prevent the stator core from forming any inter-sheet opening between outer portions of sheets during the ironing for the outer portions of the sheets.